The manufacture of methyl methacrylate is typically performed with a multi-step chemical reaction process which produces a crude product stream containing not only the desired methyl methacrylate, but also other compounds including, but not limited to, unreacted raw materials, intermediate reaction products, by-products and impurities. Thus, one of the final steps in the multi-step process for manufacture of methyl methacrylate involves purification, such as by distillation, of the crude product stream to separate the desired methyl methacrylate product from the majority of the other compounds. It has been recognized that, instead of being discarded as waste, at least some of the other compounds may be useful for production of additional quantities of methyl methacrylate or other valuable compounds if they can be further separated and either recycled to one or more steps of the multi-step process, or otherwise subjected to further reaction. Efforts have been directed at developing technologies effective and efficient for separating and recovering such other compounds including, but not limited to, methacrylic acid, 2-methacrylamide, methyl α-hydroxyisobutyrate (α-MOB).
As described in U.S. Pat. Nos. 4,529,816 and 5,393,918, the general reaction steps in a typical multi-step reaction process to produce methyl methacrylate (MMA) from acetone cyanohydrin (ACH) and sulfuric acid, are hydrolysis, cracking, esterification, separation and purification (hereinafter referred to as the “conventional ACH route to MMA” or the “conventional MMA process”). In the first reaction step, ACH is hydrolyzed with sulfuric acid to produce a hydrolysis mixture comprising α-sulfatoisobutyramide (SIBAM), α-hydroxyisobutyramide (HIBAM) and 2-methacrylamide (MAM). In a second step, the hydrolysis mixture is subjected to heating to “crack” the SIBAM and HIBAM, thereby forming a cracking reactor product comprising additional MAM. The esterification step follows, in which the cracking reactor product is combined with one or more C1-C30 alkanols and the MAM is esterified to form MMA product contained in an esterification product stream, along with MAA, MAM and α-MOB. The esterification product stream is subjected to separation and purification steps to isolate the MMA product from other compounds. Typically, one or more distillation steps are performed to produce a purified MMA product stream and a residual bottoms stream comprising other compounds including, but not limited to, MMA dimer (5-methyl-2-methylene adipate), MAA, MAM, α-MOB, methyl β-hydroxyisobutyrate (β-MOB), methyl β-methoxyisobutyrate (β-MEMOB) and methyl α-methoxyisobutyrate (α-MEMOB). The recovery and conversion of one or more of these other compounds to produce additional MMA product has been the subject of various research and development efforts having varying degrees of success and practical utility.
In JP 52010214, MMA dimer is recovered from a residual stream, derived from distillation of crude MMA and which contained 80-90 wt % MMA dimer. The residual stream was “washed” with an aqueous caustic wash to purify the MMA dimer by removing MAA, MAM, MAA dimer, MAA adducts, oxazines and other impurities from the residual stream.
In JP 52012127, a residual bottoms stream derived from MMA distillation and containing α-MOB, β-MEMOB, and MAA, was combined with the esterification aqueous effluent or esterification “acid residue” (containing mainly NH4HSO4, sulfuric acid and water) and methanol, whereupon the MMA content of the resulting mixture increased. However, it was not specified how much of the additional MMA was produced from α-MOB dehydration alone, and how much, if any, resulted from esterification of the MAA with the methanol.
FR 2064583 and GB 1256288 both disclose purification of a crude MMA product stream to produce a residual bottoms stream containing MOB and MAA, both of which are then converted to MMA by treatment with sulfuric acid and methanol, respectively and concurrently. The conversion of MOB and MAA is performed separate and apart from the hydrolysis and esterification reaction steps of the conventional MMA process, and is followed by distillation to separate the converted MMA.
The present invention seeks to provide one or more techniques for the separation and recovery of at least MAA and MAM from a residual MMA distillation stream derived, as well as conversion of one, or both, of the recovered MAA and MAM to additional MMA product.